1. Field of the Invention
The invention relates to an electrochemical cell and more particularly refers to a new and improved electrochemical storage cell or battery based on alkali metal and sulfur with at least one anode chamber and one cathode chamber which are separated from each other by an ion-conducting solid electrolyte.
2. Description of the Prior Art
U.S. Pat. No. 4,018,969 relates to an electrochemical storage cell or battery, operable in the range of about 100.degree. to 200.degree. C., of the type using an alkali metal as the anolyte and sulfur as the catholyte. The cell has at least one anode chamber and one cathode chamber which are separated from each other by an alkali ion-conducting solid electrolyte. At least one organic aprotic solvent with a boiling point above the operating temperature of the cell is contained in the cathode chamber, for at least partially dissolving the sulfur and/or its alkali compounds.
Normally, the operating temperature of alkali metal-sulfur cells is at about 300.degree. to 350.degree. C. One reason therefor is the fact that at this temperature, the conductivity of the alkali ion-conducting solid electrolyte is substantially higher than at lower temperatures. The second reason is that sulfur or alkali polysulfide is used as the cathodic reaction partner, which must be present in the molten condition. These mostly used sodium polysulfides have melting points between 242.degree. and 1200.degree. C.
At operating temperatures of 300.degree. to 350.degree. C., the discharge reaction can proceed, if sodium is used, to about Na.sub.2 S.sub.3 (more exactly, according to the phase diagram, to Na.sub.2 S.sub.2.8). This corresponds to a theoretical energy density of 760 Wh/kg. If the cell is discharged further, the reaction products Na.sub.2 S.sub.2 and Na.sub.2 S are generated, which are solid at 300.degree. to 350.degree. C. In the presence of solid reaction products, the kinetics become so poor that the cell can then be charged and discharged no longer or only with a very small power density, so that the possible higher energy densities (Na.sub.2 S corresponds to 1260 Wh/kg) cannot be obtained.
Through the presence of a solvent, the reaction can continue to proceed in the direction toward the more alkali-rich sulfides. The high melting point of such alkali-rich sulfides does not matter, since the sulfides are dissolved, at least partially, and the alkali metal is present in ion form, which ensures adequate reaction kinetics. If sodium is used, the discharge reaction can be carried out until the stoichiometry in the cathode chamber is Na.sub.2 S instead of only Na.sub.2 S.sub.3. If one neglects the mass and volume of the added solvent, the theoretical energy density is increased from 760 Wh/kg to 1260 Wh/kg. In addition, lowering the operating temperature to 100.degree. or 200.degree. C. has the advantage of substantially reduced danger of corrosion and permits the use of plastics as the housing part.
According to U.S. Pat. No. 4,018,969, the weight ratio of solvent to sulfur or alkali-sulfur compound can be between 1:10 and 1:1. The amount of solvent is preferably chosen so that up to 75% by weight of the compounds are present in the undissolved state, so as not to lower the energy density and the reaction rate unnecessarily by excessive amounts of solvent. Otherwise, it is recommended that several solvents which are mixed with each other be placed into the cathode chamber to dissolve the different alkali polysulfides and the sulfur itself.
The present invention relates to an improvement and further embodiment of the electrochemical storage cell or battery described in U.S. Pat. No. 4,018,969, and more particularly is directed to making available a solvent component in such cell especially suitable for dissolving polysulfides. It is assumed that, preferably, at least two different solvents are used.
Essentially the following four criteria have been determined for the selection of suitable solvents:
The solvent should have good solubility for the different alkali polysulfides and at the same time, good compatibility with the less polar-structured solvents suitable for dissolving sulfur as for example disclosed in related application referred to in Cross Reference to Related Application. The solvent with its content of dissolved alkali polysulfide must exhibit satisfactory conductivity. It should have a low dissociation voltage. Especially good chemical long-term stability of the solvent are furthermore important.
For extended operating times, it has now been found that the last-mentioned point is more critical than expected in some solvents which appeared to be basically suitable. Thus, multivalent alcohols or thioalcohols were entirely satisfactory for shorter and medium operating times, but not for the planned operating times of up to 5 years, as there, the decomposition would proceed on too large a scale.
The selection of suitable solvents good chemical stability for a long term is particularly difficult, especially for dissolving the polysulfides, because the presence of polar groups in the solvent molecule is necessary. The reason for this is that the polar groups make the solvent molecules susceptible to the sulfur, where the attack of the sulfur sets in at the C--H bonds, which bonds are present in the organic solvents.